Makefile: Add targets to create and validate Boot Configuration

[zmrow]

Description of changes:

    This change adds two new targets to the Makefile.  Both make use of the
    `bootconfig` tool in the latest version of the SDK.  The first new
    target, `boot-config`, gives users the ability to create a properly
    formatted Boot Configuration initrd given a valid config file.  The
    second new target `validate-boot-config`, validates the Boot
    Configuration initrd, listing its contents.  If the initrd is somehow in
    a bad format, the tool will fail and print an error.

    The Makefile targets expect a valid configuration file in the root of
    the Bottlerocket repo named "bootconfig-input".  The Boot Configuration
    initrd will be created in the root of the repo with the name
    "bootconfig.data", which is what Bottlerocket expects the file to be
    named when using it.

Testing done:
Create and validate new Boot Configuration:

$ cat bootconfig-input 
kernel {
    console = tty0, "ttyS1,115200n8"
}
init {
    systemd.log_level = debug
}

$ cargo make boot-config
...
[cargo-make] INFO - Running Task: boot-config
Creating a new boot config from input /home/fedora/bottlerocket/bottlerocket/bootconfig-input
Apply /tmp/bootconfig-input to /tmp/bootconfig.data
        Number of nodes: 8
        Size: 88 bytes
        Checksum: 6790
Boot configuration initrd may be found at /home/fedora/bottlerocket/bottlerocket/bootconfig.data
...

$ cargo make validate-boot-config
...
[cargo-make] INFO - Running Task: validate-boot-config
kernel.console = "tty0", "ttyS1,115200n8"
init.systemd.log_level = "debug"
...

Update the existing configuration:

$ cat bootconfig-input 
kernel {
    console = tty0
}

$ cargo make boot-config
...
Boot config exists at '/home/fedora/bottlerocket/bottlerocket/bootconfig.data', updating it with input /home/fedora/bottlerocket/bottlerocket/bootconfig-input
Apply /tmp/bootconfig-input to /tmp/bootconfig.data
        Number of nodes: 3
        Size: 31 bytes
        Checksum: 2360
Boot configuration initrd may be found at /home/fedora/bottlerocket/bottlerocket/bootconfig.data

$ cat bootconfig.data 
kernel {
    console = tty0
}
...

$ cargo make validate-boot-config
...
[cargo-make] INFO - Running Task: validate-boot-config
kernel.console = "tty0"

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[etungsten]

I don't think it's possible to override these variables with cargo make -e. But maybe that's ok.

[zmrow]

Good point 🤔

IMHO, I think that's probably ok considering we want to name the initrd a specific way. I suppose flexibility in the input file name would be nice, but I'm not sure how much it gets us.